It also marks the beginning of "Windows as a service" — the idea that the operating system is not just one product but a continually updated one that works across devices. Microsoft said in its announcement that "once a Windows device is upgraded to Windows 10, Microsoft will continue to keep it current for the supported lifetime of the device — at no cost".

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Microsoft is giving away Windows 10 for free apparently to reduce "fragmentation", the challenges for developers who at the moment must make software to work on a variety of operating systems. It also brings it in line with competitors like Apple, which has given away the most recent versions of its own desktop operating system.

The company said that those "who want to be among the first in line for the free Windows 10 upgrade can reserve a free copy in the coming weeks". To do so, an icon will come up in the system tray at the bottom right hand corner of the screen — clicking on it will bring up the reservation process.